ArcGIS stores and manages geographic data in a number of formats. The three basic data models that ArcGIS uses are vector, raster, and TIN. You can also import tabular data into a GIS.
Vector models
One way of representing geographic phenomena is with points, lines, and polygons. This kind of representation of the world is generically called a vector data model. Vector models are particularly useful for representing and storing discrete features such as buildings, pipes, or parcel boundaries.
Points are pairs of x,y coordinates. Lines are sets of coordinates that define a shape. Polygons are sets of coordinates defining boundaries that enclose areas. Coordinates are most often pairs (x,y) or triplets (x,y,z, where z represents a value such as elevation). The coordinate values depend on the geographic coordinate system the data is stored in. Coordinate systems are discussed in more detail in Chapter 6, ‘Preparing data for analysis’.
ArcGIS stores vector data in feature classes and collections of topologically related feature classes. The attributes associated with the features are stored in data tables.
Raster models
In a raster model, the world is represented as a surface that is divided into a regular grid of cells. The x,y coordinates of at least one corner of the raster are known, so it can be located in geographic space.
Raster models are useful for storing and analyzing data that is continuous across an area. Each cell contains a value that can represent membership in a class or category, a measurement, or an interpreted value.
Raster data includes images and grids. Images, such as an aerial photograph, a satellite image, or a scanned map, are often used for generating GIS data.
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